1. Field of the Invention
The present invention relates to a high-frequency antenna system for use in a nuclear magnetic resonance tomography device, and in particular to such an antenna system for generating and/or receiving high-frequency signals.
2. Description of the Prior Art
High-frequency antenna systems for generating and/or receiving high frequency signals in a nuclear magnetic resonance tomography device are known in the art including at least two conductor members which are supplied with current of opposite polarity and being of a predetermined length extending on an imaginary cylindrical generated surface parallel to the direction of the cylinder axis. Such an antenna arrangement is described, for example, in German OS No. 31 33 432 in combination with an additional envelope. The structure and use of the additional envelope plays no part in the subject matter of the present invention.
In supra-conductive whole-body magnets having field strengths of the basic field in the range of from about 0.5 through about 2.0T, the nuclear resonance frequencies for protons are in the range from about 20 through about 85 MHz. The magnetic high-frequency fields required for such devices can only be imperfectly generated at this frequency range with pure inductances (coils), because the wavelength becomes increasingly closer to the conductor length of the coils with increasing frequency. Phase shifts of the fields thereby result, between the beginning of the conductor and the end of the conductor of the coils. These phase shifts correspondingly reduce the functionability of the coil generating the high-frequency field. For this reason, in conventional devices, resonators are used for generating the magnetic high-frequency field. Such resonators, in the simplest case, are formed by the aforementioned two parallel oppositely supplied conductor members.
Line resonators for the given range are suggested, for example, by H. J. Schneider and P. Dullenkopf in "Review Of Scientific Instruments," Vol. 48, 68 (1977) and in the aforementioned German OS No. 31 33 432. In all cases, however, an electric field is present at the ends of the resonators described therein, this electric field penetrating the subject being examined and therefore resulting in undesired dielectric losses which deteriorate the signal-to-noise ratio, and thereby require higher high-frequency power. Moreover, the magnetic field is not limited to the examination chamber but extends far beyond the ends of the resonator, this also resulting in undesired losses and potentially resulting in noise signals from undesired regions during imaging.